We are investigating genetic factors underlying the regression of Rous sarcoma virus induced tumors in crosses of inbred lines of chickens. In the F2 generation of crosses of two inbred lines, L locus genotypes differed significantly in their ability to influence tumor regression. Similar studies at the B locus are underway. The proposed research has three objectives: First, to further investigate the role of genes at the B blood group-histocompatibility locus and the L blood group locus on tumor development using back crosses of F1's of inbred lines to the pure parental lines. Second, to determine whether or not the L locus is a minor histocompatibility locus using skin grafting technique and whether it controls the mixed leukocyte reaction. Third, to identify the chemical component(s) that is coating the lymphocyte and which renders it incapable of performing its immunological function. Acridine orange staining and electrophoretic technique will be the methods used.